A super-fast ‘light switch’ for future cars and computers

Self-driving cars have become better and more reliable in recent years. Before they might be allowed to drive completely autonomously on our roads in the near future, however, a few hurdles have to be taken. Above all, the need to assess the surroundings at lightning speed and to recognize people and obstacles takes current technologies to its limits. A team of scientists led by Juerg Leuthold at the Institute for Electromagnetic Fields at ETH Zurich, together with colleagues at the National Institute of Standards and Technology (NIST) in the USA and at Chalmers University in Gothenburg (Sweden), has now developed a novel electro-opto-mechanical switch that might be able to elegantly solve both problems in the future.

Plasmonics as a magic ingredient

To achieve this, the researchers used a magic ingredient known as “plasmonics”. In this technology, light waves are squeezed into structures that are much smaller than the wavelength of the light – which, according to the laws of optics, should be impossible to do. It can be made possible, however, by guiding the light along the boundary between a metal and a dielectric – a substance, such as air or glass, that hardly conducts electric current.

The electromagnetic waves of the light partially penetrate the metal and cause the electrons inside it to oscillate, which results in a hybrid creature made of a light wave and an electronic excitation – the plasmon. More than ten years ago, some well-known physicists already predicted that optical switches based on plasmons could lead to a revolution in data transmission and data processing, as both can be done much faster with photons than with traditional electronics.

So far, however, real-life commercial applications have failed because of the large losses encountered when transporting photons through plasmonic devices, and because of the high switching voltages needed.

Exploiting the strengths of plasmonics

“We have now solved those problems by exploiting the good properties of plasmonics while minimizing the bad ones”, says postdoc Christian Haffner, who led the project and is also first author of the recently published Science paper. The central feature of the electro-opto-mechanical switch developed by Haffner and his colleagues is a gold membrane that is only 40 nanometres thick and a few micrometres wide, and which is separated from a silicon substrate by an aluminium oxide disk.

In this configuration, the size of the gap between the gold membrane and the substrate can be controlled through mechanical forces. When a voltage is applied, the membrane bends slightly and, as a result, the gap becomes smaller.

The size of the gap, in turn, decides whether a light wave simply passes by the gold membrane or is deflected around it. This is where the plasmons come in. In fact, for a certain width of the gap only plasmons having a particular wavelength can be excited on the gold membrane. If the light has a different wavelength, it doesn’t couple to the membrane but simply propagates in a straight line inside the silicon waveguide.

Small losses and switching voltage

“Because we only use the plasmons for the short trip around the switching membrane, we have substantially lower losses than those of current electro-optic switches”, Haffner explains. “Also, we made the gold membrane very small and thin, so that we can switch it very fast and with a small voltage.”

The scientists have already demonstrated that their new switch can be flicked on and off several million times per second with an electric voltage of little more than one volt. This makes the bulky and power-hungry amplifiers typically used for electro-optical switches superfluous. In the future, the scientists plan to improve their switch further by making the gap between gold and silicon smaller still. This will make it possible to significantly reduce both the light losses and the switching voltage.

Applications from cars to quantum technologies

Possible applications for the new switch are plentiful. For instance, LIDAR systems (“Light Detection and Ranging”) for self-driving cars, in which the intensity and direction of propagation of light beams needs to be varied extremely quickly, could benefit from the fast and compact switches.

Moreover, the pattern recognition necessary for steering the cars could also be accelerated with such switches. To that end, the switches could be used in optical neural networks that mimic the human brain. There, they would be employed as weighting elements with which the network “learns” to recognize certain objects – practically at the speed of light.

Such optical implementations of circuits that normally work with electric current are also hot topics in other areas. Optical quantum circuits are also intensively studied, for instance, for the realization of quantum technologies. Until now, optical quantum circuits have been supported by classical optical switches. Those switches are typically based on a variation in the refractive index of a material when it is heated, which changes the degree to which light beams are bent by it.

However, this is a slow process and, in the long run, incompatible with the low temperatures at which other quantum elements such as the quantum bits or “qubits” of a quantum computer (corresponding to the classical bits that represent “0” and “1”) typically work. A fast switch that practically doesn’t heat up at all should, therefore, be a welcome addition to such applications, too.